The present invention is directed to radio receivers. It finds particular application in the type of radio receivers used for electromagnetic surveillance.
One type of radio receiver that is particularly effective in electromagnetic surveillance is the compressive receiver. A compressive receiver essentially performs a Fourier transformation on a band of input frequencies. It receives the band of input frequencies and repeatedly sweeps through the band. The output of the compressive receiver for a given-frequency input is a pulse of oscillations at the compressive-receiver center frequency. The time during the compressive-receiver sweep at which the pulse occurs represents the frequency of the input signal that gave rise to it. Accordingly, one can determine the frequencies at which transmissions are occuring by noting the times at which pulses occur in the compressive-receiver output. Conventional superheterodyne receivers can then be tuned to the frequencies indicated by the times at which the pulses occur, and the contents of all transmissions in the frequency band of interest can be monitored.
Although this arrangement is quite effective, it has features in which improvement may be desired. For example, an electromagnetic spectrum densely populated with transmissions requires a large number of superheterodyne receivers to provide complete monitoring of all the transmissions. This can result in a bulky, expensive surveillance apparatus.
In theory, the separate superheterodyne receivers are not needed; the pulses at the output of the compressive receiver retain phase and amplitude information. Consequently, if one produced output pulses for a frequency band of interest at a rate at least as high as the Nyquist rate for that band, a bandpass filter having the center frequency of the compressive-receiver delay line and the bandwith of the band of interest could convert the pulses to a frequency-translated version of the signal component within that band. This band could then be readily demodulated or otherwise processed in any desired manner.
Previously proposed schemes for achieving such a result, however, present certain practical problems. In order to obtain 100% time coverage of the received signals and to provide samples at a rate consistent with the resolution of the compressive receiver, its frequency range had to be restricted considerably, to less than the bandwidth of the dispresive delay lines that it employs. To obtain a wider bandwidth, it was proposed to use several parallel compressive receivers with frequency ranges wider than the bandwidths of their dispersive delay lines. The compressive receivers would have sweeps successively staggered in time, and the outputs of compressive receivers with successive sweeps would be the successive samples for the subsequent reconstruction of the received signal component. In this arrangement, the "dead" time of each receiver--which dead time is an inevitable result when a compressive receiver has a frequency range wider than the bandwidth of its delay line--is covered by one or more of the other receivers so that together the receivers provide 100% time coverage.
Although this arrangement theoretically produces the desired result of 100% time coverage in a wide-band receiver, it presents a practical problem of its own. Since information is contained in the phases of the output pulses, the phase shifts in the several compressive receivers have to track each other, and it is quite difficult to provide compensation circuitry with the ability to maintain the required degree of phase tracking.
It is accordingly an object of the present invention to monitor a frequency in which a compressive receiver has detected a transmission but to avoid having to use a separate superheterodyne receiver for each detected frequency.
It is another object of the present invention to enable a wide-band compressive receiver to eliminate dead periods for all frequencies within its frequency range.
It is a further object of the present invention to provide an improved compressive-receiver surveillance system.